The present invention pertains to a process for breaking and separating cylindrical particles of a wide length distribution into fractions preferably having a specific length to diameter ratio. Physical blends of granular pesticidal products are becoming increasingly important in agriculture as a simple means to customize product offerings without costly registration of multiple formulations (as described in WO 9700608 A1).
U.S. Pat. No. 3,549,014 issued Dec. 22, 1990, discloses a device for screening pulverulent material comprising sifting or screening means having substantially the shape of a surface of revolution or of a drum. WO-A 97 00608 discloses a homogeneous mixture of cylindrical granules of one or more groups having substantially uniform diameters and a longitudinal length of no more than 8 times the diameter with the average length being 1.5 to 4 times the diameter but gives no guidance on how to consistently produce such a size distribution.
Preventing or at least minimizing segregation of these physical blends is critical. When making substantially homogeneous (i.e. uniform) blends, admixture of cylindrical granules as opposed to spherical granules is particularly advantageous. A substantially uniform blend is formed when two or more cylindrically shaped granular materials have a similar and relatively narrow range of average lengths.
To minimize segregation in a blend of cylindrical granules, the granules have preferably substantially the same diameter. As cylindrical pesticidal granules are typically produced by an extrusion process, the diameter is defined by the orifices in the die which can be chosen at will and thus matched precisely.
Further in regard to minimizing segregation in a blend of particles such as cylindrical granules, the lengths of the granules are preferably matched as closely as possible. If the lengths of the resulting "as-made" granules are sufficiently different, post granulation processing is necessary to ensure homogeneity.
The predominate extrusion method for agricultural pesticides is paste extrusion. With paste extrusion, the moistened formulation is forced through a die and the strands of extrudate are simply allowed to break off naturally into random lengths. The extrudate is typically dried in a fluid bed device wherein the strands can break down further but are still of widely varying length. The breadth and median size of the length distribution can vary with each formulation.
The other extrusion method used for agricultural pesticides is melt extrusion wherein heated formulation is forced through a die and the incipient strands of extrudate are cut to consistent length at the dieface. With this method, both diameter and length are controlled and the "as-made" granules are uniformly sized. However, melt extruded formulations are not yet widespread in the agricultural industry and the known method for controlled sizing of paste extruded granules is not particularly effective.
It is an objective of this invention to provide a process for post-granulation sizing of particles such as extruded granules, especially paste extruded granules, to obtain a relatively narrow range of length to diameter ratios.
It is a further objective to perform said sizing with high conversion to the desired length so that there are few fines which require recycle to the granulation step.
It is a further objective to provide an apparatus which is capable to separate the desired product cut from the unwanted sizes of particles.
It is a further objective to provide a process which is controllable and can provide desired length to diameter ratios and length profiles on a wide range of formulations with differing initial granule length distributions.
Another objective is to define a particle composition obtainable by use of the apparatus according to the invention.